telnet - User interface to the TELNET protocol
telnet [-d] [-a] [-n tracefile] [-e escapechar] [[-l user] host [port]]
The telnet command is used to communicate with another host using the TELNET protocol. If telnet is invoked without the host argument, it enters command mode, indicated by its prompt (telnet>). In this mode, it accepts and executes the commands listed below. If it is invoked with arguments, it performs an open command with those arguments.
Once a connection has been opened, telnet will attempt to enable the TELNET LINEMODE option. If this fails, then telnet will revert to one of two input modes: either ``character at a time'' or ``old line by line'' depending on what the remote system supports.
When LINEMODE is enabled, character processing is done on the local system, under the control of the remote system. When input editing or character echoing is to be disabled, the remote system will relay that information. The remote system will also relay changes to any special characters that happen on the remote system, so that they can take effect on the local system.
In ``character at a time'' mode, most text typed is immediately sent to the remote host for processing.
In ``old line by line'' mode, all text is echoed locally, and (normally) only completed lines are sent to the remote host. The ``local echo character'' (initially ``^E'') may be used to turn off and on the local echo (this would mostly be used to enter passwords without the password being echoed).
If the LINEMODE option is enabled, or if the localchars toggle is TRUE (the default for ``old line by line``; see below), the user's quit, intr, and flush characters are trapped locally, and sent as TELNET protocol sequences to the remote side. If LINEMODE has ever been enabled, then the user's susp and eof are also sent as TELNET protocol sequences, and quit is sent as a TELNET ABORT instead of BREAK There are options (see toggle autoflush and toggle autosynch below) which cause this action to flush subsequent output to the terminal (until the remote host acknowledges the TELNET sequence) and flush previous terminal input (in the case of quit and intr).
While connected to a remote host, telnet command mode may be entered by typing the telnet ``escape character'' (initially ``^]''). When in command mode, the normal terminal editing conventions are available.
The following telnet commands are available. Only enough of each command to uniquely identify it need be typed (this is also true for arguments to the mode, set, toggle, unset, slc, environ, and display commands).
display argument ...
Displays all, or some, of the set and toggle values (see below).
mode type Type is one of several options, depending on the state of the TELNET session. The remote host is asked for permission to go into the requested mode. If the remote host is capable of entering that mode, the requested mode will be entered.
isig (-isig) Attempt to enable (disable) the TRAPSIG mode of the LINEMODE option. This requires that the LINEMODE option be enabled.
edit (-edit) Attempt to enable (disable) the EDIT mode of the LINEMODE option. This requires that the LINEMODE option be enabled.
Attempt to enable (disable) the SOFT_TAB mode of the LINEMODE option. This requires that the LINEMODE option be enabled.
Attempt to enable (disable) the LIT_ECHO mode of the LINEMODE option. This requires that the LINEMODE option be enabled.
open host [[-l] user][-port]
Open a connection to the named host. If no port number is specified, telnet will attempt to contact a TELNET server at the default port. The host specification may be either a host name (see hosts(5)) or an Internet address specified in the ``dot notation'' (see inet(3)). The [-l] option may be used to specify the user name to be passed to the remote system via the ENVIRON option. When connecting to a non-standard port, telnet omits any automatic initiation of TELNET options. When the port number is preceeded by a minus sign, the inital option negotiation is done. After establishing a connection, the file .telnetrc in the users home directory is opened. Lines begining with a # are comment lines. Blank lines are ignored. Lines that begin without whitespace are the start of a machine entry. The first thing on the line is the name of the machine that is being connected to. The rest of the line, and successive lines that begin with whitespace are assumed to be telnet commands and are processed as if they had been typed in manually to the telnet command prompt.
Sends one or more special character sequences to the remote host. The following are the arguments which may be specified (more than one argument may be specified at a time):
escape Sends the current telnet escape character (initially ``^'').
If the remote side supports the TELNET STATUS command, getstatus will send the subnegotiation to request that the server send its current option status.
rently running process.
synch Sends the TELNET SYNCH sequence. This sequence causes the remote system to discard all previously typed (but not yet read) input. This sequence is sent as TCP urgent data (and may not work if the remote system is a 4.2BSD system -- if it doesn't work, a lower case ``r'' may be echoed on the terminal).
set argument value
unset argument value
The set command will set any one of a number of telnet variables to a specific value or to TRUE. The special value off turns off the function associated with the variable, this is equivalent to using the unset command. The unset command will disable or set to FALSE any of the specified functions. The values of variables may be interrogated with the display command. The variables which may be set or unset, but not toggled, are listed here. In addition, any of the variables for the toggle command may be explicitly set or unset using the set and unset commands.
escape This is the telnet escape character (initially ``^['') which causes entry into telnet command mode (when connected to a remote system).
If telnet is in localchars mode (see toggle localchars below) and the flushoutput character is typed, a TELNET AO sequence (see send ao above) is sent to the remote host. The initial value for the flush character is taken to be the terminal's flush character.
If telnet is in localchars mode (see toggle localchars below) and the interrupt character is typed, a TELNET IP sequence (see send ip above) is sent to the remote host. The initial value for the interrupt character
is taken to be the terminal's intr character.
If telnet is operating in LINEMODE or ``old line by line`` mode, then this character is taken to be the terminal's reprint character. The initial value for the reprint character is taken to be the terminal's reprint character.
Thi is the file to which the output, caused by netdata or option tracing being TRUE, will be written. If it is set to ``-'', then tracing information will be written to standard output (the default).
If telnet is operating in LINEMODE or ``old line by line`` mode, then this character is taken to be the terminal's worderase character. The initial value for the worderase character is taken to be the terminal's worderase character.
slc state The slc command (Set Local Characters) is used to set or change the state of the the special characters when the TELNET LINEMODE option has been enabled. Special characters are characters that get mapped to TELNET commands sequences (like ip or quit) or line editing characters (like erase and kill).
By default, the local special characters are exported.
The environ command is used to manipulate the the variables that my be sent through the TELNET ENVIRON option. The initial set of variables is taken from the users environment, with only the DISPLAY and PRINTER variables being exported by default. The USER variable is also exported if the -a or -l options are used.
Valid arguments for the environ command are:
define variable value
Define the variable variable to have a value of value. Any variables defined by this command are automatically exported. The value may be enclosed in single or double quotes so that tabs and spaces may be included.
Remove variable from the list of environment variables.
Mark the variable variable to be exported to the remote side.
Mark the variable variable to not be exported unless explicitly asked for by the remote side.
toggle arguments ...
Toggle (between TRUE and FALSE) various flags that control how telnet responds to events. These flags may be set explicitly to TRUE or FALSE using the set and unset commands listed above. More than one argument may be specified. The state of these flags may be interrogated with the display command.
Valid arguments are:
susp, see send above).
Execute a single command in a subshell on the local system. If command is ommitted, then an interactive subshell is invoked.
Get help. With no arguments, telnet prints a help summary. If a command is specified, telnet will print the help information for just that command.
Telnet uses at least the HOME, SHELL, DISPLAY, and TERM environent variables. Other envirnoment variables may be propogated to the other side via the TELNET ENVIRON option.
~/.telnetrc user customized telnet startup values
The Telnet command appeared in 4.2BSD.
On some remote systems, echo has to be turned off manually when in ``old line by line'' mode.
In ``old line by line'' mode or LINEMODE the terminal's eof character is only recognized (and sent to the remote system) when it is the first character on a line.
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